To meet higher capacity demands and to enhance user experience, cellular communications network are increasing the number of base stations employed. One approach for increasing the density of base stations is achieved by cell splitting macro cells in highly loaded geographical areas. Specifically, the macro cell may be split into multiple small cells in highly loaded geographical areas. Another approach for meeting high capacity demands is to employ a mixture of macro cells and small cells with overlapping coverage areas within the cellular network. This type of cellular network may be referred to as heterogeneous networks (HetNets). Such networks may be an important complement to macro cell splitting. One example includes a cellular network having clusters of pico cells within the macro coverage area to offload macro traffic. A pico base station provides service to a pico cell. Typically, a pico base station is a low power node (LPN) that transmits with low output power and covers a much smaller geographical area than a high power node, such as a macro base station. Other examples of low power nodes are home base stations and relays. This densification of the underlying support for the cellular network may allow radio resources to be reused. Additionally, because wireless devices may be closer to the serving base station, wireless devices may achieve higher bitrates. However, while the presence of additional base stations may increase system performance and improve user experience, inter-cell interference may be more pronounced.
Multiple-input multiple output (MIMO) systems employed at both the transmitter and the receiver may also be used to improve communication performance. Specifically, multiple antennas may be used at both the transmitter and the receiver to perform smart antenna functions. Such functions may include spreading the total transmit power over the antennas to achieve an array gain that incrementally improves the spectral efficiency (more bits per second per hertz of bandwidth) and/or achieves a diversity gain that improves the link reliability by reducing facing. In modern communication networks, MIMO is an essential element of wireless communication standards such as IEEE 802.11n (Wi-Fi), IEEE 802.11ac (Wi-Fi) 4G, 3GPP Long Term Evolution, WiMAX, and HSPA+.
In LTE MIMO systems, data is transmitted using transport blocks transmitted over a Physical Downlink Shared Channel (PDSCH). At the physical layer, transport blocks are converted into codewords. There are a number of steps involved in the conversion, depending on the length of the transport block. For example, a 24 bit checksum (CRC) may be appended to the transport block. This CRC may be used to determine whether the transmission was successful or not and may trigger Hybrid ARQ to send an ACK or NACK, as appropriate. As another example, the transport block may be segmented into code blocks, which may be between 40 and 6144 bits long. Additionally, the conversion may include the reassembling of the resulting code blocks into a single code word, which may be considered a transport block with error protection.
A wireless device that includes multiple antennas may be configured to receive multiple code words in a single transmission interval. For example, two code words can be independently transmitted on two or more transmit antennas over the same radio resources by mapping them on to two or more transmission layers. For example, in LTE open-loop spatial multiplexing, a.k.a. Transmission Mode 3 (TM3), when the channel conditions support multiple rank transmission, two code words may be transmitted using large delay cyclic delay diversity (CDD) over two layers. At the receiving wireless device, which may also be termed user equipment (UE), the two code words can be detected by receiving the signal using multiple receive antennas. Though these techniques are applicable to TM3, TM3 is just one example. The described techniques are equally applicable to any multi-input/multi-output transceiver mechanisms.
Typically the transmission parameters, such as the modulation and coding scheme (MCS), transmit power, and other parameters are determined and/or adapted by the network node at every transmission instant (every TTI or multiple TTIs) for a future downlink (DL) transmission based on the receiver performance feedback information received from the wireless device. In the case of TM3, there is one common Channel Quality Indicator (CQI) feedback for both the code words transmitted. However, the receive status of each code word (i.e. ACK/NACK feedback) may be transmitted over Physical Uplink Control Channel (PUCCH) or Physical Uplink Shared Channel (PUSCH) separately based on the wireless device category and the feedback setting. In one example, the network node may maintain two independent sets of transmission parameters for each code word. This will help to improve the overall spectral efficiency if the two code words are transmitted over two or more layers.
In a M×N MIMO system with M transmit antennas and N receive antennas, each code word may be encoded by a forward error correction (FEC) and symbol mapped based on the transmission parameter adjustments (i.e., modulation encoding scheme (MCS)) recommended by the link adaptation module. The precoding vector can be changed every TTI in a predefined way in accordance with the 3GPP LTE standards. The receiving wireless device typically estimates the channel rank and estimates the CQI. The code word receive status is decided after checking the CRC of each of the code word. The transmission parameters for the subsequent down link (DL) transmissions, for example MCS can be adjusted based on the CQI feedback and also considering the ACK/NACK status for each code word. However, the adjustment based on ACK/NACK feedback is typically slow.
Furthermore, and as discussed above with regard to heterogeneous networks, significant interference may be experienced from the neighboring cells. At the wireless device, each code word experiences different levels of inter-cell interference in a typical heterogeneous deployment even with a careful planning. Additionally, the average interference power experienced over the two data resource elements that are carrying the two code words may be significantly different due to path loss incurred over the link. These factors may result in the code words being associated with differing error rates. As a result, different ACK/NACKs may be received for a pair of code words.